1. Field of the Invention
The present invention relates to a positive photoresist composition which exhibits a high sensitivity and a high definition and has improved focal depth range properties and underexposure margin.
The present invention also relates to a positive photoresist composition which can satisfactorily image a pattern in a process using a 0.2- to 0.8-xcexcm thin film.
2. Description of the Related Art
In production of very large scale integrated circuits (VLSIS) where a high definition of not more than half a micron, particularly of not more than 0.35 xcexcm is required, demands have been made to provide photoresist compositions which exhibit satisfactory sensitivity, definition, underexposure margin, and focal depth range properties and can image resist patterns with good shapes.
The term xe2x80x9cunderexposure marginxe2x80x9d as used herein is the ratio Eop/Es, where Eop is the exposure to reproduce a pattern in exact accordance with predetermined dimensions of a mask pattern, and Es is the minimum exposure to image an isolated pattern. The higher the underexposure margin is, the higher the margin, and is preferred.
Japanese Patent Laid-Open No. 6-167805 (conventional technology 1) discloses a high-definition positive photoresist composition including, as a photosensitizer, a quinonediazide ester having a linear tetranuclear to heptanuclear polyphenol skeleton. This photosensitizer exhibits a high definition but has a very low underexposure margin for its very high y value. The photosensitizer therefore tends to fail to image patterns in areas where exposure is low (underexposure side).
In the exposure step, therefore, underexposure occurs and the imaging of a resist pattern is blocked when the exposure time period is only several milliseconds less than the optimum exposure (exposure Eop) or when the thickness of a resist film becomes only several hundred nanometers thicker at locations corresponding to steps of a substrate. Thus, satisfactory resist patterns cannot be imaged.
In addition, a material containing the photosensitizer in question cannot image a pattern when the focus of a light source shifts to the plus side (i.e., the light focus is present on the substrate side from the surface of the resist) upon exposure in the imaging of a resist pattern with a fine line-and-space (LandS) of not more than 0.35 xcexcm (FIG. 1(a)). This is a peculiar characteristic of this type of photosensitizer as compared with a material containing a photosensitizer including no linear tetranuclear to heptanuclear skeleton, such as a quinonediazide ester of a compound shown by the following formula (i) (FIG. 1(b)). 
As a result, the photosensitizer is disadvantageous in that it is narrow in focal depth range and cannot image a resist pattern of the predetermined dimensions when the distance between a light source and a substrate shifts in the exposure step or when a resist film is formed on a stepped substrate and hence the surface of resist film is uneven due to steps of the substrate.
Japanese Patent Laid-Open No. 10-213906 (conventional technology 2) discloses a positive photoresist composition including a compound shown by the following formula (ii): 
wherein each of R17 to R24 is independently a hydrogen, a halogen, a hydroxyl group, an alkyl group, an alkoxy group, an aryl group, or a nitro group; each of R25 and R26 is independently a hydrogen, a halogen, an alkyl group, an aryl group, a nitro group, xe2x80x94(CH2)nxe2x80x94OR27, or xe2x80x94(CH2)nxe2x80x94COOR28, where R27 is a hydrogen, an alkyl group, an aryl group, or an alkanoyl group, R28 is a hydrogen, an alkyl group, or an aryl group, and n is an integer of 0 to 3, and a quinonediazide ester of a phenol compound shown by the following formula (i): 
The publication states that this composition can inhibit reduction in film thickness of a resist and exhibits satisfactory definition and focal depth range properties.
The positive photoresist composition, however, exhibits narrow or low underexposure margin and focal depth range properties in the imaging of a resist pattern with a fine LandS of not more than 0.35 xcexcm.
Separately, a photolithography technique using an i-ray (365 nm) cannot significantly image an ultrafine resist pattern of not more than 0.35 xcexcm, particularly of not more than 0.30 xcexcm. Accordingly, the uses of a light source (e.g., KrF and ArF) having a shorter wavelength than the i-ray and of an expensive chemically amplified photoresist material have been proposed.
However, an exposure system for excimer laser such as KrF or ArF is a very expensive system, and the chemically amplified resist material used with this type of system is also expensive, which invites increased production costs.
Accordingly, demands have been made to prolong the use of the conventional photolithography technique using i-ray. When an ultrafine resist pattern of not more than 0.35 xcexcm, and particularly of not more than 0.30 xcexcm is to be formed using i-ray, a photosensitive film should be advantageously a thin film. However, a thin film of the photosensitive film of not more than 1.0 xcexcm, particularly of not more than 0.8 xcexcm cannot sufficiently absorb light energy upon exposure, and cannot inhibit the interference of light waves. A fine resist pattern cannot therefore be imaged. Even if a resist pattern is imaged, the film thickness in unexposed areas is reduced or a residue (scum) after development is formed at the bottom of a resist pattern, inviting defective etching of the underlayer substrate.
Accordingly, it is an object of the present invention to provide a positive photoresist composition which exhibits a high sensitivity and a high definition and has improved focal depth range properties and underexposure margin in the production of very large scale integrated circuits (VLSIs) where a high definition of not more than 0.35 xcexcm is required.
Another object of the present invention is to provide a positive photoresist composition which can image an ultrafine resist pattern of not more than 0.30 xcexcm in a process using a 0.2- to 0.8-xcexcm thin film.
After intensive investigations to achieve the above objects, the present inventors found that an invented positive photoresist composition mentioned below can image a resist pattern having satisfactory focal depth range properties and underexposure margin even in the imaging of an ultrafine resist pattern with a fine LandS of not more than 0.35 xcexcm.
They also found that the invented positive photoresist composition can image a satisfactory resist pattern even in the imaging of an ultrafine resist pattern of not more than 0.30 xcexcm in a process using a 0.2- to 0.8-xcexcm thin film. The invention has been accomplished based on these findings.
Specifically, the present invention provides, in an aspect, a positive photoresist composition. The composition includes (A) an alkali-soluble resin, (B) a photosensitizer including a quinonediazide ester of at least one selected from compounds shown by the following formula (I): 
wherein each of R1 to R8 is independently a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a cycloalkyl group; each of R9, R10, and R11 is independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; Q is a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or is combined with R9 to form a carbocycle having 3 to 6 carbon atoms, or is a residue shown by the following formula (II); each of a and b is an integer of 1 to 3; d is an integer of 0 to 3, and n is an integer of 0 to 3: 
wherein each of R12 and R13 is independently a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a cycloalkyl group; and c is an integer of 1 to 3; and (C) a compound having a gram absorption coefficient of 5 to 60 with respect to light with a wavelength of 365 nm.
In the positive photoresist composition, the ingredient (A) is preferably an alkali-soluble novolak resin obtained synthetically by a condensation reaction of a phenol compound ingredient with an aldehyde or a ketone, and the phenol compound ingredient includes 1 to 20% by mole of a phenol compound shown by the following formula (III): 
wherein R14 is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, 30 to 95% by mole of m-cresol, and 2 to 50% by mole of 2,5-xylenol.
The phenol compound ingredient for use in the synthesis of the ingredient (A) may further include 1 to 30% by mole of at least one phenol compound selected from p-cresol, 3,4-xylenol, 3,5-xylenol, and dimethylol derivatives of these compounds.
In the positive photoresist composition, the phenol compound ingredient for use in the synthesis of the ingredient (A) may include, instead of 2,5-xylenol, 1 to 25% by mole of a phenol compound shown by the following formula (IV): 
wherein e is 0 or 1.
The ingredient (C) in the positive photoresist composition may be 2,2xe2x80x2,4,4xe2x80x2-tetrahydroxybenzophenone having a gram absorption coefficient of 47.4.
The ingredient (C) may be a compound shown by the following formula (V) having a gram absorption coefficient of 29.3. 
The ingredient (C) may also be a compound shown by the following formula (VI) having a gram absorption coefficient of 27.4. 
Alternatively, the ingredient (C) may be a compound shown by the following formula (VII) having a gram absorption coefficient of 18.8. 
The invention provides, in another aspect, a substrate with a photosensitive film. The substrate with a photosensitive film is obtained by applying a film of the aforementioned positive photoresist composition onto a substrate, and drying the applied film to form a photosensitive film having a thickness of 0.2 to 0.8 xcexcm on the substrate.
In a further aspect, the invention provides a process for imaging a resist pattern. This process includes the step of subjecting the aforementioned substrate with a photosensitive film to a selective exposure with i-ray (365 nm) to image a resist pattern.